Project Summary It is estimated that nearly 1.5 million new cases of cancer are reported each year. Until recently, treatment options for metastatic cancers have been limited. However, therapies that harness the activity of the host immune system have proven successful in the treatment of recalcitrant cancers such as melanoma, acute lymphoblastic leukemia and Hodgkin lymphoma. Numerous immune cell lineages regulate anti-tumor responses, including Batf3-dependent dendritic cells, which cross-present antigens to nave CD8 T cells and initiate adaptive immune responses against cancer cells. Therefore, research to identify the factors that regulate the development and function of dendritic cells can advance our understanding of immune responses to cancer and reveal putative molecular targets for cancer immunotherapy. The present proposal aims to define the mechanism by which the transcription factor, LMYC, regulates DC function and in turn whether LMYC is required for steady state and immunotherapy-induced tumor rejection. We have generated Mycl1-GFP knockin/knockout mice that serve as in vivo reporters of Mycl1 expression and Mycl1-deficiency. Using this model, we have shown previously that Mycl1 is required for optimal priming of nave CD8 T cells during infections. Given that both CD8 T cells and Batf3-dependent dendritic cells are required for tumor rejection and positive responses to immunotherapy, we hypothesize that Mycl1 is required for these processes. In Aim1, we will define the molecular mechanism by which LMYC supports the core function of Batf3-dependent dendritic cells. We will first determine the impact of LMYC deficiency on global transcription to identify pathways that are dysregulated at steady state. In vivo and in vitro biochemical analyses will be employed to validate that broad defects in transcription result in downstream effects on core biological processes, such as protein synthesis and respiration, which are broadly regulated by MYC family members. In Aim2, we will determine whether steady state and immunotherapy-induced rejection of immunogenic tumors is dependent on intrinsic expression of Mycl1 by Batf3-dependent dendritic cells. Simultaneously, we will analyze the impact of the tumor microenvironment and immunotherapies on the expression of Mycl1 by dendritic cells in vivo. If Mycl1 expression correlates with rejection outcomes in a mouse model of checkpoint blockade, it may serve as a molecular biomarker to predict patient responses to cancer immunotherapy. Therefore, the results of this study will advance our understanding of the molecular mechanisms that control dendritic cell function as well as the molecules required for steady state and immunotherapy-induced immune responses to cancer.